The present invention relates to multicomponent superabsorbent gel particles containing at least one acidic water-absorbing resin and at least one basic water-absorbing resin. Each superabsorbent gel particle has microdomains of the acidic resin and/or basic resin homogeneously dispersed throughout the gel particle.
Water-absorbing resins are widely used in sanitary goods, hygienic goods, wiping cloths, water-retaining agents, dehydrating agents, sludge coagulants, disposable towels and bath mats, disposable door mats, thickening agents, disposable litter mats for pets, condensation-preventing agents, and release control agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethyl-cellulose, crosslinked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidines, and polyacrylonitriles.
Such water-absorbing resins are termed xe2x80x9csuperabsorbent polymers,xe2x80x9d or SAPs, and typically are lightly crosslinked hydrophilic polymers. SAPs are generally discussed in Goldman et al. U.S. Pat. No. 5,669,894. SAPs can differ in their chemical identity, but all SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. For example, SAPs can absorb one hundred times their own weight, or more, of distilled water. The ability to absorb aqueous fluids under a confining pressure is an important requirement for an SAP used in a hygienic article, like a diaper.
The dramatic swelling and absorbent properties of SAPs are attributed to (a) electrostatic repulsion between the charges along the polymer chains, and (b) osmotic pressure of the counter ions. It is known, however, that these absorption properties are drastically reduced in solutions containing electrolytes, such as saline, urine, and blood. The polymers do not function as effective SAPs in the presence of such physiologic fluids.
The decreased absorbency of electrolyte-containing liquids is illustrated by the absorption properties of a typical, commercially available SAP, i.e., sodium polyacrylate, in deionized water and in 0.9% by weight sodium chloride (NaCl) solution. The sodium polyacrylate can absorb 146.2 grams (g) of deionized water per gram of SAP (g/g) at 0 psi (g/g), 103.8 g of deionized water per gram of polymer at 0.28 psi, and 34.3 g of deionized water per gram of polymer of 0.7 psi. In contrast, the same sodium polyacrylate is capable of absorbing only 43.5 g, 29.7 g, and 24.8 g of 0.9% aqueous NaCl at 0 psi, 0.28 psi, and 0.7 psi, respectively. The absorption capacity of SAPs for body fluids, like urine or menses, therefore, is dramatically lower than for deionized water because such fluids contain electrolytes. This dramatic decrease in absorption is termed xe2x80x9csalt poisoning.xe2x80x9d
The salt poisoning effect has been explained as follows. Water-absorption and water-retention characteristics of SAPs are attributed to the presence of ionizable functional groups in the polymer structure. The ionizable groups typically are carboxyl groups, a high proportion of which are in the salt form when the polymer is dry, and which undergo dissociation and salvation upon contact with water. In the dissociated state, the polymer chain contains a plurality of functional groups having the same electric charge and, thus, repel one another. This electronic repulsion leads to expansion of the polymer structure, which, in turn, permits further absorption of water molecules. Polymer expansion, however, is limited by the crosslinks in the polymer structure, which are present in a sufficient number to prevent solubilization of the polymer.
It is theorized that the presence of a significant concentration of electrolytes interferes with dissociation of the ionizable functional groups, and leads to the xe2x80x9csalt poisoningxe2x80x9d effect. Dissolved ions, such as sodium and chloride ions, therefore, have two effects on SAP gels. The ions screen the polymer charges and the ions eliminate the osmotic imbalance due to the presence of counter ions inside and outside of the gel. The dissolved ions, therefore, effectively convert an ionic gel into a nonionic gel, and swelling properties are lost.
The most commonly used SAP for absorbing electrolyte-containing liquids, like urine, is neutralized polyacrylic acid, i.e., containing at least 50%, and up to 100%, neutralized carboxyl groups. Neutralized polyacrylic acid, however, is susceptible to salt poisoning. Therefore, to provide an SAP that is less susceptible to salt poisoning, either an SAP different from neutralized polyacrylic acid must be developed, or the neutralized polyacrylic acid must be modified or treated to at least partially overcome the salt poisoning effect.
The removal of ions from electrolyte-containing solutions is often accomplished using ion exchange resins. In this process, deionization is performed by contacting an electrolyte-containing solution with two different types of ion exchange resins, i.e., an anion exchange-resin and a cation exchange resin. The most common deionization procedure uses an acid resin (i.e., cation exchange) and a base resin (i.e., anion exchange). The two-step reaction for deionization is illustrated with respect to the desalinization of water as follows:
NaCl+Rxe2x80x94SO3Hxe2x86x92Rxe2x80x94SO3Na+HCl 
HCl+Rxe2x80x94N(CH3)3 OHxe2x86x92Rxe2x80x94N(CH3)3 Cl+H2O. 
The acid resin (Rxe2x80x94SO3H) removes the sodium ion; and the base resin (Rxe2x80x94N(CH3)3OH) removes the chloride ions. This ion exchange reaction, therefore, produces water as sodium chloride is adsorbed onto the resins.
The most efficient ion exchange occurs when strong acid and strong base resins are employed. However, weak acid and weak base resins also can be used to deionize saline solutions. The efficiency of various combinations of acid and base exchange resins are as follows:
Strong acidxe2x80x94strong base (most efficient)
Weak acidxe2x80x94strong base
Strong acidxe2x80x94weak base
Weak acidxe2x80x94weak base (least efficient).
The weak acid/weak base resin combination requires that a xe2x80x9cmixed bedxe2x80x9d configuration be used to obtain deionization. The strong acid/strong base resin combination does not necessarily require a mixed bead configuration to deionize water. Deionization also can be achieved by sequentially passing the electrolyte-containing solution through a strong acid resin and strong base resin.
A xe2x80x9cmixed bedxe2x80x9d configuration is simply a physical mixture of an acid ion exchange resin and a base ion exchange resin in an ion exchange column, as disclosed in Battaerd U.S. Pat. No. 3,716,481. Other patents directed to ion exchange resins having one ion exchange resin imbedded in a second ion exchange resin are Hatch U.S. Pat. No. 3,957,698, Wade et al. U.S. Pat. No. 4,139,499, Eppinger et al. U.S. Pat. No. 4,229,545; and Pilkington U.S. Pat. No. 4,378,439. Composite ion exchange resins also are disclosed in Hatch U.S. Pat. Nos. 3,041,092 and 3,332,890, and Weiss U.S. Pat. No. 3,645,922.
The above patents are directed to nonswelling resins that can be used to remove ions from aqueous fluids, and thereby provide purified water. Ion exchange resins used for water purification must not absorb significant amounts of water because resin swelling resulting from absorption can lead to bursting of the ion exchange containment column.
Ion exchange resins or fibers also have been disclosed for use in absorbent personal care devices (e.g., diapers) to control the pH of fluids that reach the skin, as set forth in Berg et al., U.S. Pat. No. 4,685,909. The ion exchange resin is used in this application to reduce diaper rash, but the ion exchange resin is not significantly water absorbent and, therefore, does not improve the absorption and retention properties of the diaper.
Ion exchange resins having a composite particle containing acid and base ion exchange particles embedded together in a matrix resin, or having acid and base ion exchange particles adjacent to one another in a particle that is free of a matrix resin is disclosed in B. A. Bolto et al., J. Polymer Sci.:xe2x80x94Symposium No. 55, John Wiley and Sons, Inc. (1976), pages 87-94. The Bolto et al. publication is directed to improving the reaction rates of ion exchange resins for water purification and does not utilize resins that absorb substantial amounts of water.
Other investigators have attempted to counteract the salt poisoning effect and thereby improve the performance of SAPs with respect to absorbing electrolyte-containing liquids, such as menses and urine. For example, Tanaka et al. U.S. Pat. No. 5,274,018 discloses an SAP composition comprising a swellable hydrophilic polymer, like polyacrylic acid, and an amount of an ionizable surfactant sufficient to form at least a monolayer of surfactant on the polymer. In another embodiment, a cationic gel, like a gel containing quaternized ammonium groups and in the hydroxide (i.e., OH) form, is admixed with an anionic gel (i.e., a polyacrylic acid) to remove electrolytes from the solution by ion exchange. Quaternized ammonium group in the hydroxide form are very difficult and time-consuming to manufacture, thereby limiting the practical use of such cationic gels.
Wong U.S. Pat. No. 4,818,598 discloses the addition of a fibrous anion exchange material, like DEAE cellulose, to a hydrogel, like a polyacrylate, to improve absorption properties. The ion exchange resin xe2x80x9cpretreatsxe2x80x9d the saline solution (e.g., urine) as the solution flows through an absorbent structure (e.g., a diaper). This pretreatment removes a portion of the salt from the saline. The conventional SAP present in the absorbent structure then absorbs the treated saline more efficiently than untreated saline. The ion exchange resin, per se, does not absorb the saline solution, but merely helps overcome the xe2x80x9csalt poisoningxe2x80x9d effect.
WO 96/17681 discloses admixing discrete anionic SAP particles, like polyacrylic acid, with discrete polysaccharide-based cationic SAP particles to overcome the salt poisoning effect. Similarly, WO 96/15163 discloses combining a cationic SAP having at least 20% of the functional groups in a basic (i.e., OH) form with a cationic exchanger resin, i.e., a nonswelling ion exchange resin, having at least 50% of the functional groups in the acid form. WO 96/15180 discloses an absorbent material comprising an anionic SAP, e.g., a polyacrylic acid and an anion exchange resin, i.e., a nonswelling ion exchange resin.
These references disclose combinations that attempt to overcome the salt poisoning effect. However, the references merely teach the admixture of two types of particles, and do not suggest a single particle containing individual microdomains of an acidic resin and/or a basic resin. It would be desirable to provide discrete SAP particles that exhibit exceptional water absorption and retention properties, especially with respect to electrolyte-containing liquids, and thereby overcome the salt poisoning effect.
The present invention is directed to multicomponent SAPs comprising at least one acidic water-absorbing resin, like a polyacrylic acid, and at least one basic water-absorbing resin, like a poly(dialkylaminoalkyl acrylamide) or a poly(dialkylaminoalkyl methacrylamide), hereafter collectively referred to as poly(dialkylaminoalkyl (meth)acrylamides).
More particularly, the present invention is directed to multicomponent SAP particles containing discrete microdomains of at least one acidic water-absorbing resin and/or at least one basic water-absorbing resin homogeneously dispersed throughout the particle. The acidic resin can be a strong or a weak acidic resin. Similarly, the basic resin can be a strong or a weak basic resin. A preferred SAP contains microdomains of at least one weak acidic resin and/or at least one weak base resin.
Accordingly, one aspect of the present invention is to provide SAP particles that overcome the salt poisoning effect and demonstrate an improved ability to absorb and retain electrolyte-containing liquids, like saline, blood, urine, and menses.
Another aspect of the present invention is to provide an SAP having improved absorption and retention properties compared to a conventional SAP, like sodium polyacrylate. The present SAP particles are produced, for example, by coextruding an acidic water-absorbing hydrogel and a basic water-absorbing hydrogel to provide multicomponent SAP particles having a plurality discrete microdomains of an acidic resin and a basic resin dispersed throughout the particle. Such SAP particles demonstrate improved absorption and retention properties compared to SAP compositions comprising a simple admixture of acidic resin particles and basic resin particles.
In accordance with another important aspect of the present invention, the present multicomponent SAP particles also can be prepared by admixing dry particles of a basic resin with a hydrogel of an acidic resin, then extruding the resulting mixture to form multicomponent SAP particles having microdomains of a basic resin dispersed throughout a continuous phase of an acidic resin. Alternatively, dry acidic resin particles can be admixed with a basic resin hydrogel, followed by extruding the resulting mixture to form multicomponent SAP particles having microdomains of an acidic resin dispersed in a continuous phase of a basic resin. In addition, a multicomponent SAP particle containing microdomains of an acidic resin and a basic resin dispersed in a continuous phase of a matrix resin can be prepared by adding dry particles of the acidic resin and dry particles of the basic resin to a hydrogel of the matrix hydrogel, then extruding.
In accordance with yet another important aspect of the present invention, the acidic and basic resins are lightly crosslinked, such as with a suitable polyfunctional vinyl polymer. In preferred embodiments, the acidic resin, the basic resin, and/or the entire multicomponent SAP particle are surface treated to further improve water absorption and retention properties, especially under a load.
Yet another important feature of the present invention is to provide an SAP particle containing microdomains of a weak acidic water-absorbing resin and/or a weak basic water-absorbing resin. An example of a weak acid resin is polyacrylic acid having 0% to 20% neutralized carboxylic acid groups (i.e., DN=0 to DN=20). Examples of weak basic water-absorbing resins are a poly(vinylamine), a polyethylenimine, and a poly(dialkylaminoalkyl (meth)acrylamide) prepared from a monomer either having the general structure formula (I) 
or the ester analog of (I) having the general structure formula (II) 
wherein R1 and R2, independently, are selected from the group consisting of hydrogen and methyl, Y is a divalent straight chain or branched organic radical having 1 to 8 carbon atoms, and R3 and R4, independently, are alkyl radicals having 1 to 4 carbon atoms. An example of a strong basic water-absorbing resin is poly(vinylguanidine).
These and other aspects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments.